Explosive growth of the internet and the worldwide web is driving a need for increased communication data rates. In the corporate world, the need for high-speed access or data rates is met by dedicated high-speed links (perhaps T1/E1 frame relays or OC1 ATM systems) from the company to an internet access provider. Users in the company utilize a local area network (LAN) to gain access to an internet access router, which is attached to the high-speed link. Unfortunately, home-users of the internet do not often have a high-speed link and must rely on standard analog or plain old telephone service (POTS) line.
The need for high-speed access to the home is ever increasing due to the availability of information, data, programs, entertainment, and other computer applications on the worldwide web and on the internet. For example, designers of web technology are constantly developing new ways to provide sensory experiences, including audio and video, to users of the web (web surfers). Higher-speed modems will be required so the home-user can fully interact with incoming web and communication technologies.
Although designers of modems are continuously attempting to increase data rates, analog or POTS line modems are presently only able to reach data rates of up to 56 kilobits per second (Kbps). These conventional analog modems transmit and receive information on POTS subscriber lines through the public-switched telephone network. The internet access provider is also coupled to the switched telephone network and transmits and receives information through it to the subscriber line.
Some home-users have utilized integrated digital services network (ISDN) equipment and subscriptions to obtain up to 128 Kbps access or data rates by the use of 2 B channels. ISDN equipment and subscriptions can be expensive and require a dedicated subscriber line. Heretofore, neither ISDN modems nor analog modems are capable of providing 256 Kbps or higher access between the home and the internet. Over one megabit per second (Mbps) data rates with analog modems or ISDN equipment do not seem feasible at this time.
A variety of communication technologies are competing to provide high-speed access to the home. For example, asymmetric digital subscriber lines (ADSL), cable modems, satellite broadcast, wireless LANs, and direct fiber connections to the home have all been suggested. Of these technologies, the asymmetric digital subscriber line can utilize the POTS subscriber line (the wire currently being utilized for POTS) between the home-user (the residence) and the telephone company (the central office).
ADSL networks and protocols were developed in the early 1990's to allow telephone companies to provide video-on-demand service over the same wires which were being used to provide POTS. ADSL technologies include discrete multitone (DMT), carrierless amplitude and phase modulation (CAP), VHDL, and other technologies. Although the video-on-demand market has been less than originally expected, telephone companies have recognized the potential application of ADSL technology for internet access and have begun limited offerings.
ADSL technology allows telephone companies to offer high-speed internet access, while removing internet traffic from the telephone switch network. Telephone companies cannot significantly profit from internet traffic in the telephone switch network due to regulatory considerations. However, the telephone company can charge a separate access fee for ADSL services. The separate fee is not as restricted by regulatory considerations.
With reference to FIG. 1, a conventional asymmetric ADSL (ADSL) system 10 includes a copper twisted pair analog subscriber line 12, an ADSL modem 14, an ADSL modem 16, a band splitter 18, and a band splitter 20. Line 12 is a POTS local loop or wire connecting a central office 32 of the telephone company to a user's residence 22.
ADSL modem 14 is located in user's residence 22 and provides data to and from subscriber line 12. The data can be provided from line 12 through modem 14 to various equipment (not shown) coupled to modem 14. Equipment, such as, computers, network devices, servers, or other devices, can be attached to modem 14. Modem 14 communicates with a data network (not shown) coupled to modem 16 across line 12. ADSL modem 16 receives and transmits signals from and to line 12 to modem 14. The data network can be coupled to other networks (not shown), including the internet.
At least one analog telephone 26, located in residence 22, can be coupled to subscriber line 12 through splitter 20 for communication across line 12 with telephone switch network 28. Telephone 26 and telephone switch network 28 (e.g., public-switched telephone (PST) network) are conventional systems well-known in the art. Alternatively, other analog equipment, such as, facsimile machines, POTS modems, answering machines, and other telephonic equipment, can be coupled to line 12 in user's residence 22.
System 10 requires that band splitter 18 and band splitter 20 be utilized to separate higher frequency ADSL signals and lower frequency POTS signals. For example, when the user makes a call from residence 22 on telephone 26, lower frequency signals (under 4 kilohertz (kHz)) are provided through band splitter 20 to subscriber line 12 and through band splitter 18 to telephone switch network 28 in central office 32. Band splitter 18 prevents the lower frequency POTS signals from reaching ADSL modem 16. Similarly, band splitter 20 prevents any of the POTS signals from reaching modem 14.
ADSL modem 16 and ADSL modem 14 communicate higher frequency ADSL signals across subscriber line 12. The higher frequency ADSL signals are prevented from reaching telephone 26 and telephone switch network 28 by band splitters 20 and 18, respectively. Splitters 18 and 20 can be passive analog filters or other devices which separate lower frequency POTS signals (below 4 kHz) from higher frequency ADSL signals (above 50 kHz).
The separation of the POTS signals and the ADSL signals by splitters 18 and 20 is necessary to preserve POTS voice and data traffic and ADSL data traffic. More particularly, splitters 18 and 20 can eliminate various effects associated with POTS equipment which may affect the transmission of ADSL signals on subscriber line 12. For example, the impedance of subscriber line 12 can vary greatly as at least one telephone 26 is placed on-hook or off-hook. Additionally, the changes in impedance of subscriber line 12 can change the ADSL channel characteristics associated with subscriber line 12. These changes in characteristics can be particularly destructive at the higher frequencies associated with ADSL signals (e.g., from 30 kHz to 1 megahertz (MHz) or more).
Additionally, splitters 18 and 20 isolate subscriber line wiring within residence 22. The impedance of such wiring is difficult to predict. Further still, the POTS equipment, such as, telephone 26, provides a source of noise and nonlinear distortion. Noise can be caused by POTS voice traffic (e.g., shouting, loud laughter, etc.) and by POTS protocol, such as, the ringing signal. The nonlinear distortion is due to the nonlinear devices included in conventional telephones. For example, transistor and diode circuits in telephone 26 can add nonlinear distortion and cause hard clipping of ADSL signals. Telephone 26 can further generate harmonics which can reach the frequency ranges associated with the ADSL signals. The nonlinear components can also demodulate ADSL signals to cause a hiss in the audio range which affects the POTS.
Conventional ADSL technology has several significant drawbacks. First, the costs associated with ADSL services can be quite high. Telephone companies incur costs related to the purchase of central office equipment (ADSL modems and ADSL network equipment) and to the installation of such equipment. Residential users incur subscriber equipment costs (ADSL modems) and installation costs.
Installation costs are particularly expensive for the residential user because trained service personnel must travel to residence 22 to install band splitter 20 (FIG. 1). Although band splitter 18 must be installed at the central office, this cost is somewhat less because service personnel can install band splitter 18 within central office 32. Also, at office 32, splitter 18 can be included in ADSL modem 16. However, in residence 22, splitter 20 must be provided at the end of subscriber line 12.
Additionally, ADSL equipment for the residence, such as, modem 14, is expensive because the most complex component of modem 14 (e.g., the receiver) is located at residence 22, since high-speed transmissions are generally received within residence 22 (e.g., are downstream), and lower-speed transmissions are received by central office 32 (e.g., are upstream). In most internet applications, larger amounts of data are requested by the residential user rather than by the internet source. Receivers are typically much more complex than transmitters. These high-speed receivers often receive data at rates of over 6 Mbps.
ADSL equipment can also be subject to cross-talk noise from other subscriber lines situated adjacent to subscriber line 12. For example, subscriber lines are often provided in a closely contained bundle. The close containment can cause cross-talk from other subscriber lines to be placed on subscriber line 12.
More particularly, cross-talk noise from upstream traffic in high-speed communication systems, such as, system 10, can be an especially significant problem because such traffic often originates from different points on the subscriber lines. Accordingly, the signals associated with the traffic in different lines can vary greatly in amplitude at the same point. This variation in amplitude accentuates problems associated with cross-talk noise.
Heretofore, some conventional ADSL systems limit the upstream data rate (e.g., data rate from modem 14 to ADSL modem 16) and transmit at the low end of the frequency spectrum to minimize cross-talk. However, these techniques alone have not been adequate to fully compensate for cross-talk noise. One such form of cross-talk noise is near-end cross-talk (e.g., NEXT noise), which must be dealt with adequately or else data can be significantly affected. Near-end cross-talk noise is propagated in a disturbed channel in the direction opposite to the direction of propagation of the signal in the disturbing channel. The terminal of the disturbed channel at which near-end cross-talk noise is present is ordinarily close to, or coincides with, the energized terminal of the disturbing channel. Near-end cross-talk noise becomes a significant problem for modems associated with ADSL modem 16 because downstream data rates are higher and can occur at high frequency signals.
Thus, there is a need for a digital subscriber line (DSL) communication system which reduces near-end cross-talk noise. Further, there is a need for a communication system which reduces near-end cross-talk noise inexpensively, without degrading data rates. Further still, there is a need for a splitterless DSL modem which is less susceptible to errors due to cross-talk noise on the subscriber line.